I. Field of the Invention
The present invention relates to silane coatings for treating metals. More particularly, the present invention provides organofunctional silane coatings which improve the adhesion of rubber and other polymeric materials to a metal substrate.
II. Description of the Prior Art
Rubber to metal bonded components find two major areas of application which may be classified as tire and non-tire applications. In tires, traditionally brass is used as a thin coating over a steel cord for imparting adhesion to natural rubber compounds. Non-tire applications include components, such as vibration dampers on motor and engine mounts, and applications in the medical, appliance, and other industries, where basic functions such as fluid control, energy conversion, sealing, vibration isolation, and/or combinations of these functions are required. Additionally, tire-to-metal metallic reinforcement of conveyer belts and hoses is a further example of an industrial application where a rubber substrate needs to be adhesively bound to an associated metal.
Many different systems and methods have been proposed to adhere rubber, or other polymeric materials, to metals. For example, organofunctional silanes have been found to promote adhesion between a metal substrate and a polymer layer. Silanes have been particularly useful in tire applications where strong and reliable bonds are critical. However, adhesion or bond strength is typically dependent upon the particular silane or combination of silanes, and the particular system upon which the silanes are used to coat the metal. In other words, the amount of adhesion provided by a particular silane coating typically depends on the metal substrate as well as the polymer layer to be adhered thereto. For example, while certain silanes may provide improved adhesion between the metal substrate and a peroxide-cured rubber, these same silanes will often not provide the same results for sulfur-cured rubber.
Hydrophobic silanes, such as alkoxy silanes, have been proposed to provide good bond strength between rubbers and metal. However, formulations of such silane solutions require flammable organic solvents for dissolution, leading to more viscous solutions and thicker final coatings. Such coatings are difficult to apply, cure, and control, particularly for coating uniformity, and need to be dried, prior to bonding to the rubber, at elevated temperatures, such as about 160° C. which pose additional hazards from flammable solvents. Bond strength with these silane coatings are predominantly optimal with coating thicknesses of at least 1 μm. Such coatings are not very amenable to industrial applications, and tire-cord applications in particular, where efficiency in time and cost is important to the success of the products.
Further, it has been difficult to bond rubbers to metal substrates utilizing various silanes and combinations thereof, and especially in those instances in which the adhesively-bound surfaces are subjected to fuels, oils, and/or other organic solvents, as the silanes may react therewith and/or dissolve therein to weaken or degrade the adhesive bond. Solvent-based adhesive systems are used in non-tire applications for bonding metals to rubbers. However, solvent-based adhesives are flammable, and hence hazardous, generate high levels of volatile organic compounds and vapors, and pose environmental disposal problems. These disadvantages render these systems less useful in industrial applications.
In rubber compounds, adhesion promoters such as cobalt salt additives and HRH systems (hexamethylenetetramine, resorcinol and hydrated silica) are used to further enhance rubber adhesion to metals, and particularly for tire cords. More specifically, cobalt additives improve the aged adhesion and dynamic adhesion properties of the rubber-brass bond. Although the performance of the rubber-to-metal bond is adequate for most applications, inclusion of such promoters presents drawbacks. For example, cobalt salts are expensive and pose availability problems. Further, cobalt has been found to affect the rubber properties upon aging, and in particular, cobalt causes accelerated degradation of the rubber.
Rubber to metal bonded applications in tires have further drawbacks. For example, the brass plating process sometimes involves treatment with hazardous chemicals, such as cyanide. Additionally, brass is prone to galvanic erosion in conjunction with the underlying steel, and furthermore, the brass composition typically requires a copper content of about 63-68% which imposes restrictions on the rubber composition. These limitations further require consideration when selecting an appropriate adhesive and/or bonding method for bonding rubber, or other tire materials, to the metal tire cord.
Thus, there is a need for compositions and methods for improving the adhesion between a metal substrate and a polymeric material, such as rubber. There is a further need to provide the bond without the use of promoters in the polymeric material, and in particular, without the use of cobalt. There is yet a further need to provide a bonding method that is safe and convenient, while reducing risks associated with use of hazardous solvents.